FS#2577 - distinguish between anonymous reporter and deleted user

When a user is deleted from Flyspray, their opened tasks, closed task and task comments are then shown as Anonymous Submitter, the same way as anonymous reporters (not really anonymous, just that user does not have login account, but usually their email address is stored within that task data).

Currently just the entry from users table are deleted when a user is deleted. Their internal user_id integer is still within tasks and comments fields, and maybe some other tables too. So there is not a ON DELETE SET NULL rule or something like that applied. As it is just an autoincremented number by the system, this is not personal data imho and should be no problem for GDPR, but gives Flyspray the ability to distinguish between anon reporters and deleted users. Well, we could also look if there is an email address within task table entry for notification of anonymous reporter, but there are also tasks possible that have no user_id nor an email address.

It might by useful to present that information differently like deleted user or showing the info differently like icon + title-tooltip with explanation.

Also interesting what happens with mentions of a deleted username in a comment or task description. (see FS#2322)

The user isn’t in database, but deleting that now gone user should not modify tasks or comment where that username was mentioned I think.But what if another user registers under that now gone username? In that case that new user would inherit that mentions. Probably we can ignore that edge case as there will be not much things will happen with an old mention in old tasks/comments.